Method and device for producing nano films

ABSTRACT

A method for producing films or more precisely stated, mats and plates out of nanofibers, in particular films or mats and plates out of cellulose nanofibers, wherein
         nanofiber material is dispersed in solvent;   the solids content is between 1 and 4% by volume and the resulting slurry is applied by means of an application device to a conveyor belt that is moving in the conveying direction and   then the poured-on slurry is pre-dried and   the pre-dried slurry, after the pre-drying, is subjected to the main drying and after the achievement of a predetermined moisture content, is removed from the conveyor belt.

FIELD OF THE INVENTION

The invention relates to a method for producing nanofilms and to an apparatus for doing so.

BACKGROUND OF THE INVENTION

Plastic films and in particular thin plastic films are almost universally used as packaging materials, both for shopping bags and for films used as a component in food packaging.

Particularly through packaging redemption systems, some of these foils can be brought back into the production cycle or at least put to use through incineration.

More and more, though, it is turning out to be a global problem that plastic shopping bags in particular end up in the environment and because of their lack of biodegradability, are present in the environment on a massive scale, particularly causing problems not just in the oceans, but also in other bodies of water.

In addition, disintegrating films, which end up as plastic nanoparticles, have caused a considerable concentration in bodies of water.

In more developed industrial nations, attempts are being made to combat this problem by means of packaging made from paper or also fibers. In this connection, it is disadvantageous that these are sometimes not competitive from a price standpoint and are also less desirable in other ways.

In some cases, efforts are also being made to replace plastics in the packaging sector with corn and other starch systems. In this case, however, the disadvantage is that these are not very durable in the presence of water.

AT 516198 A1 has disclosed a method for producing a product composed of a bioplastic that contains nanofibers. This is understood to be a biosynthetic that is produced from biopolymers and contains nanocellulose fibers. The method for producing the nanofiber-containing bioplastic includes applying and distributing a compound of the bioplastic with the nanofibers in a fluid state onto a surface of at least one support that is moving in a rotation direction and transporting the bioplastic compound, which has been applied to and distributed on the surface of the at least one support, in the rotation direction of the at least one support. It is also specified that the nanofiber-containing bioplastic is at least partially dried and also that the partially dried compound is detached from the surface.

US 2010/0124651 A1 has disclosed a method for producing a thin layer of nanocellulose. In this case, the layer is applied directly to the surface of a plastic backing material so that the nanocellulose fibers form a thin layer. The nanocrystalline suspension in this case is applied to an impermeable fabric belt on which a first drying process is then begun, this drying process being carried out with hot air. After the first drying process, an additional fabric layer is placed onto the top surface of the film; this fabric layer is preferably semipermeable, with the intent being to produce a sandwich effect. The semipermeable fabric layer should be able to allow liquids and gases to pass through. After the nanocrystalline film is placed between the two fabric layers, a second drying is carried out, with the three-layer composite being rolled onto a cylindrical metallic dryer. The dryer then conveys heat through the impermeable fabric; the heat causes the moisture in the nanocellulose film to evaporate and escape through the semipermeable fabric layer. The two fabric layers are needed to stabilize the nanocrystalline film. In this connection, the three-layer composite can also be conveyed, via a plurality of dryer units. Only after the second drying has the nanocrystalline film gelled to the point that the film can be dried without the impermeable backing layer. The second semipermeable backing layer is still needed, however, Only after leaving the third drying stage is the film stable enough that it can be further processed without backing layers. In this case, it is disadvantageous that this method is very complex, entails a high investment in equipment, and requires a surface treatment of the film.

US 2014/0255688 A1 has disclosed another method for producing a film out of nanocellulose; this film is likewise applied to at least one surface of a backing material. This document discusses a plurality of solvents; the correspondingly produced suspension should contain two or more precisely, 0.25-2% by weight nanocellulose. In addition, plasticizers such as glycol or sorbitol can be used.

The film is applied to the backing layer in such a way that a thickness of 50-150 μm is achieved. Then a drying is performed with hot air at 60-95° C., preferably 80° C.

In practice, it has turned out that with this method, the hot air drying causes the pores of the film to close at the surface so that it does not dry in a reliable way. In addition, this method has also not proven itself valuable in practice for the simple reason that in practice, the layer thicknesses are actually more than 150 μm and it is thus not possible to produce truly thin films.

The object of the invention is to create a method for producing films that can be used to produce environmentally friendly films based on renewable resources.

Another object is to create an apparatus for producing the films, which makes it possible to produce the films on a large industrial scale.

SUMMARY OF THE INVENTION

According to the invention, a film is made from nanocellulose. The base material nanocellulose and nanofibers made of cellulose are basically known. Up to this point, however, large-scale processing has not been carried out since efforts in this direction have met with failure so that there have only been thin layers that are manually produced, but cannot meet the requirements of a film.

In particular, thin layers of this kind have been produced manually up to this point, have not been uniformly thick, and have also been much too thick for use as film. Apart from this, thin layers of this kind were more like a paper than a film.

According to the invention, nanofibers are dissolved in a suitable solvent. Due to the special nanofibers, a suitable solvent is desired because these fibers have an activated surface. The solvent in this case can be an organic solvent, water, or a mixture thereof. In particular, it can take the form of alcohols or alcohol/water mixtures.

It has turned out that a formulation with 1 to 4% solids content achieves a good dispersibility with an appropriate density. The solids content can also be higher in certain applications.

This formulation is completely homogenized by agitating or whipping.

In this case, it is advantageous to provide at least 1,000 revolutions per minute as the speed of the agitator and to agitate this formulation for 24 hours and preferably, for 48 hours.

When using water, at least deionized water is used or optionally, distilled water.

After the corresponding dissolution of the nanofibers, they are introduced into a container used as a storage receptacle. In order to prevent sedimentation and separation, the receptacle is embodied with an appropriate agitating mechanism or the like. Among other things, gas can also be pumped through it. Basically, it is suitable to use any method that serves to prevent corresponding sedimentation in storage receptacles.

The discharge from the storage receptacle can be carried out with conventional, known metering pumps, with tubular or eccentric screw pumps being particularly suitable. Furthermore, compressed air-controlled containers can be used here. Even when supplying slurry, it is essential that no thickening, drainage, or separation takes place.

In order to produce the actual film, a pouring of slurry onto a backing material is then initiated.

In this case, the application is carried out by means of a doctor blade, which can be profiled. It is also suitable to use a bar with an indentation ground into it, a corresponding blade-shaped outlet nozzle, a slot nozzle, or a so-called slide-die nozzle, or volume-controlled application nozzles.

The layer thickness is controlled by controlling the volume or more precisely stated, controlling the nozzle opening and the nozzle gap or using the appropriate doctor blade or scraper.

Naturally, this can also be influenced by the speed at which the backing material is conveyed away.

The backing material is preferably a very smooth backing material, in particular a belt conveyor; the conveyor belt can be a metal or plastic conveyor belt.

If a metal belt is used as the belt, then a steel belt is used, for example, in particular a stainless steel belt that is corrosion-resistant.

In this case, such a metal belt has thicknesses of 0.5 to 0.35 mm and with very large widths and very large conveyor rollers, can even reach up to 1 mm thick or more.

Naturally, suitable and known plastics are also suitable, especially also PTFE belts or PTFE-coated belts.

In order to produce a thin film from highly dispersed slurry, which is runny at the time of the application, the slurry must be correspondingly dried on the belt. Because of the low solids contents, however, this is an extremely delicate process since nanofiber slurries of this kind have a very pronounced tendency to form bubbles.

In particular, it is important to match the types of drying to each other and also to adjust the rail parameters such as the speed, layer thickness, application width, and the like.

In addition to the risk of bubble formation, with these special films, there is the risk that powerful internal stresses will form, which sharply reduce the strength of the film.

According to the invention, the drying consists of three drying components.

A first component is the heated supporting belt. The belt in this case is heated with hot rollers via heating lines, with metal belts being particularly advantageous in this context.

In this case, the temperature of the belt is set to 40 to 95° C.

After the application and simultaneously to the heated belt, a pre-drying can be carried out by means of short-wave to middle-wave infrared heating elements arranged in the longitudinal and transverse directions; preferably, a battery of heating elements is provided in which the individual heating elements can be separately regulated. In this case, it is advantageous if the air supply can be regulated during the infrared drying, i.e. if a transverse or longitudinal flow is present, which can adjust a corresponding vapor pressure in the material or above the material.

To this end, the belt can be routed, in an encapsulated fashion in a space that is closed off from the outside, thus permitting a selective ventilation and permitting the heat removal of both the heat that is introduced by means of the conveyor belt and the heat that is introduced by the infrared heating elements to be carried out with the appropriate ventilation.

For the main drying, the pre-dried slurry is then acted on with middle-wave or short-wave infrared heating elements with a high power density and individual controllability. Here, too, it is extremely important to reliably remove the solvent vapor resulting from the high liquid content of 95 to 99% so as to prevent a bubble formation or a cooking or baking of the applied slurry.

To ensure this, the large quantity of air required for the drying is selectively guided toward the slurry with diffuse outflows in order not to produce any pressure on the slurry. According to the invention, it has turned out that an excessively direct guidance of air toward the material produces waves when the material is wet and produces cracks when the material is dry.

According to the invention, the material is optionally remoistened in order to establish a predetermined flexibility. A complete drying of the material causes it to become relatively rigid and inflexible so that a remoistening can be used to establish a specific flexibility.

The remoistening in this case can take place in a vapor chamber or fog chamber in which a predetermined relative humidity is set and moisture is correspondingly supplied.

After the completed drying and possible remoistening, the film is detached from the supporting bed with the aid of doctor blades and undergoes an edge trimming. The edge-trimmed, removed film is then wound onto a roll, with the rolling being performed by a torque-controlled winder drive.

With the above-indicated method, it is possible to use corresponding nanomaterials such as nanocellulose to produce a film that is not inferior to a plastic film in its usability, but is made of renewable raw materials and is biodegradable.

This film is suitable in this context both for laminates for electrical industry and for food packaging, secondary food packaging, and as a replacement for plastic bags, gloves, and the like.

The slurry here is particularly applied with a bed thickness of 1 to 20 mm and, after drying and removal, achieves a thickness of 1 to 200 μm or more. In this case, the thickness that is produced is achieved with a very high degree of uniformity; in the end, the film has a width of 1 m, for example, and it is easily possible to produce 50 m of film per minute, which approaches the realm of the commercially usable. The system can also be used to produce mats or plates of nanomaterial, with a possible slurry application of 20 to 40 mm and up, depending on the lateral boundaries.

BRIEF DESCRIPTION OF THE DRAWING

The invention and in particular, the apparatus used, will be explained by way of example based on a drawing. The sole FIGURE here shows a very schematic side view of a corresponding apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus according to the invention for producing nanofilms 1 has a dispersing unit 2; the dispersing unit 2 has a receptacle 3 and an agitating or whipping mechanism 4. In addition, the dispersing unit 2 has a supply for nanofiber material and a supply for corresponding solvents. The agitating or whipping device 4 in this case is preferably an electromotive device 4 with a corresponding agitating or whipping mechanism 5; the agitating or whipping mechanism 5 and the motor 4 are embodied so that rotation speeds of more than 1,000 revolutions per minute can be achieved and in addition, the agitating mechanism mixes and disperses the entire contents.

Sufficiently dispersed material or more precisely, the slurry produced by means of this, can be transferred to a storage receptacle 6; the storage receptacle 6 has corresponding supply devices for supplying the slurry from the dispersing unit 2 to the storage receptacle 6. The storage receptacle 6 likewise has an agitating mechanism 7, which has a drive 8 and a corresponding agitator 9; these components are dimensioned so that they keep the slurry in the dispersed, non-separated state.

By means of a hose system 10, a corresponding pump 11, and a corresponding downstream hose 12, the slurry can be correspondingly supplied to an application device 13. The application device 13 in this case is a doctor blade and in this case, extends across the width of the system. The slurry is poured from the application device 13 onto a conveyor belt 14, which moves in accordance with the arrow direction 15. To tension the conveyor belt 14, at least one first roller 16 and one last roller 17 are provided; between the rollers 16, 17, there can be a plurality of additional support and heating rollers 18. The rollers 16, 17 in this case rest with a partial circumference against the belt 14, while the other rollers 18 preferably rest against the belt 14 in a supporting fashion over only a relatively narrow region of the belt.

The rollers 16, 17, 18 are preferably all embodied as heatable, in particular electrically heatable; belt 14 temperatures of between 40 and 85° can be set.

After the application device 13, there is a pre-drying device 19; the pre-drying device 19 is composed of a plurality of IR heating elements 20 arranged longitudinally relative to the travel direction 15 of the belt 14 and a plurality of IR heating elements 21 arranged transversely relative to the travel direction of the belt 14.

Naturally, it is also possible to position the heating elements 20, 21 obliquely or at an angle to the travel direction 15 of the belt 14.

It is also possible to embody the IR heating elements at different distances from the belt surface.

In the vicinity of the pre-drying 19, it is also possible for there to be a housing (not shown) around the entire pre-drying unit 19, which shields the pre-drying unit from the outside atmosphere and the uncontrolled access by this atmosphere, but does provide a selective ventilation, e.g. a transverse ventilation by means of corresponding nozzles and corresponding opposing suction devices.

The pre-drying unit 19 is followed by the drying unit 22; the drying unit has at least one, but preferably a plurality of infrared heating elements 23, which can be arranged similarly to the heating elements of the pre-drying unit, but can also be infrared heating elements that operate in particular spots and that can preferably be individually controlled.

Preferably, the surface temperature of the slurry is measured and the heating intensity by means of the infrared heating elements can be adapted to a desired surface temperature, which can particularly vary from the entrance of the drying unit to the exit from the drying unit.

In order to set a defined vapor pressure above the slurry 24, the drying unit 22 can also have a surrounding housing, which prevents the uncontrolled access by the surrounding atmosphere, but permits an appropriate supply of air or another gas. The air or the gas, whose relative humidity is adjusted so that it is able to absorb a particular quantity of moisture from the slurry 24, is guided so that it flows through the region above the slurry 24.

In this case, it is important that the flow travels in a diffuse fashion and is not aimed directly at the surface since a direct flow against the surface can produce waves in the very moist slurry in the region of the entrance to the drying unit 22 and at the exit, can produce cracks in the slurry material, which is then already correspondingly thin and film-like.

The drying unit 22 is followed by a remoistening device 25, which can be embodied as a fog or humidity chamber or which brings about a defined remoistening in some other way.

To achieve this, it is possible to measure the moisture content of the slurry, preferably in a contactless way, and to regulate the moistening in a corresponding way.

The film 26 that is produced by means of this is removed from the last roller 17, possibly with a corresponding removing blade (not shown), and is supported by a support roller 27. It then travels over a spreader roller 28 to a winder 29, where it is correspondingly wound.

Since during the film can develop a static charge during production, after the removal of the film 26 from the conveyor belt 14, an air ionization device or another device 30 for reducing or eliminating the charge can be provided.

In order to correspondingly guide and tension the conveyor belt, it is possible for other rollers 31, particularly in the form of tension rollers 31, to be correspondingly provided in the region of the lower run of the belt.

The invention has the advantage that for the first time, it is possible in a reproducible, high-quality method to produce a nanofiber film with a high degree of uniformity with regard to the thickness over both its length and width; the production is carried out in widths of 1 m and more and at speeds of 50 m per minute, which enables the method to be used commercially. 

1. A method for producing films or mats and plates out of cellulose nanofibers, comprising: dispersing nanofiber material in solvent, resulting in a slurry having a solids content between 1 and 4% by volume; pouring the resulting slurry onto a conveyor belt that is moving in a conveying direction; pre-drying the slurry that has been poured on the conveyor belt using infrared heating elements that are arranged longitudinally and/or transversely or diagonally to the conveying direction of the conveyor belt; after the pre-drying, subjecting the pre-dried slurry to a main drying performed with middle-wave and/or short-wave infrared heating elements with a high power density, wherein the infrared heating elements are controlled individually or in groups, and selectively removing solvent vapor and removing moisture in the main drying by supplying air and/or other gases that are diffusely directed into a region of the main drying; and after achieving a predetermined moisture content, removing the slurry from the conveyor belt.
 2. The method according to claim 1, wherein the nanofiber material is dispersed in a solvent selected from at least one of the group consisting of: water an organic solvent, and alcohol.
 3. The method according to claim 1, wherein the slurry is poured onto the conveyor belt and applied using a doctor blade, a profiled doctor blade, a bar with an indentation ground into it, a blade-shaped nozzle, a slot nozzle, a slide-die nozzle, or a volume-controlled application nozzle.
 4. The method according to claim 1, wherein the slurry is poured onto a smooth conveyor belt.
 5. The method according to claim 1, comprising heating the conveyor belt to 40 to 95° C.
 6. The method according to claim 1, comprising carrying out the pre-drying in a housing with a controlled gas supply or air supply.
 7. The method according to claim 1, wherein the infrared heating elements are individually controllable.
 8. The method according to claim 1, comprising adjusting a gas flow or air flow during the main drying with regard to a relative humidity and/or flow speed of the slurry.
 9. The method according to claim 1, wherein after the main drying, the dried slurry material is remoistened to a desired moisture content.
 10. A device for performing a method for producing films from cellulose nanofibers, comprising: a conveyor belt; a plurality of rollers for conveying the conveyor belt along a longitudinal direction in a loop in the form of an upper and lower run; an application device for applying a film slurry onto a region of the conveyor belt; a pre-drying unit, following the application device in a conveying direction of the conveyor belt, wherein the pre-drying unit comprises infrared heating elements that act on the poured-on slurry; and following the pre-drying unit in the conveying direction of the conveyor belt, a main drying unit with infrared heating elements; wherein in the region of the conveyor belt in which the conveyor belt supports the slurry, the conveyor belt is held by rollers, at least some of the rollers are heatable.
 11. The device according to claim 10, wherein the main drying unit is followed in the conveying direction by a remoistening unit.
 12. The device according to claim 10, further comprising a dispersing device, which comprises a receptacle and an agitating device; wherein the agitating device can agitate at more than 1,000 revolutions per minute. 